Kate’s blue dress became so popular, the “SNL” wardrobe department had to create its own for a skit with Anne Hathaway. Photo: EPA

Kate is Great — for fashion, that is. Ever since Kate Middleton and Prince William revealed their engagement on Nov. 16, her clothes, her jewelry and even her haircut have been sending the masses — both in the UK and the US — into a royal frenzy.

The blue Issa London dress she wore on the day of the announcement sold out worldwide within 24 hours, and spawned an equally successful knockoff by British supermarket Tesco. Her coatmaker, Katherine Hooker, has been besieged by inquiries. Her sapphire engagement ring, once owned by William’s late mother, Princess Diana, is now sparking a sales surge among jewelers.

“Business has been really crazy,” says Michael Arnstein, CEO of The Natural Sapphire Company in Midtown. “Nobody has left the office. We have breakfast, lunch and dinner in here. A few weeks ago, we had 15 employees, and today we’ll have 23 staffers to keep up with the outrageous demand.”

That Kate, a 28-year-old commoner with few major designer labels in her wardrobe, could suddenly be the new Michelle Obama comes as no surprise to pop-culture experts.

“There’s something so middle-of-the-road about Kate that she can appeal to everyone,” says Rob Shuter, a British-born columnist for Web site Popeater.

“A nice married mother in Ohio can appreciate her the same way a girl who works at Vogue can.”

JERSEY DRESSES

Never before have a few yards of jersey fabric proved so popular. The $615 Issa London wrap dress sported by Kate as she announced her engagement sold out worldwide 24 hours later.

Even the crew of “Saturday Night Live” couldn’t get the midnight-blue frock for a Nov. 20 skit, in which Anne Hathaway played an eager-to-please Miss Middleton. “Our wardrobe department made the dress,” says a show insider.

In Britain, national supermarket chain Tesco created a $26 knockoff, which sold out online within an hour.

For New Yorkers still interested in the real thing, Internet retailer Net-A-Porter started a waiting list on Nov. 24. Half the site’s order has already been reserved, so get your name on the list now.

SAPPHIRE RINGS

Kate’s 18-carat sapphire engagement ring is making the world feel blue. The style, named “The Lady Di,” has been popular since the 1980s, but Kate has increased demand for sapphires to an even greater degree, says Michael Arnstein, the CEO of The Natural Sapphire Company in Midtown.

“We’ve sold well over 100 [sapphire engagement] rings. I’m anticipating we’re going to sell every sapphire we own before Christmas, which is unprecedented,” says Arnstein, whose rings start at $1,000 and go up to $95,625.

But phony crown jewels are having a moment, too. Costume jewelry kingpin Kenneth Jay Lane has designed a $39.54 ring with a “simulated sapphire,” which debuted on QVC Nov. 20. Since then, more than 30,000 units have sold, says Tim Byrket, the director of merchandising for QVC. “Not since the movie ‘Titanic’ has a piece of jewelry been knocked off so much,” jokes Shuter.

FLIRTY HATS

Given the gorgeous brunette’s penchant for fascinators and whimsical toppers (she favors London milliner Philip Treacy’s creations), New York-based hatmakers are also gearing up for a busy spring.

“We’re very excited about the announcement because we imagine it will introduce a new generation of women to wearing feathers, fascinators and hats,” says Linda Pagan, spokesperson for New York-based Milliners Guild. “As we get closer to the wedding, we’ll see people get into the mood. We are definitely expecting a bump. April is already our biggest month, but we’re already looking forward to an even busier April,” says Pagan, who also owns The Hat Shop in SoHo.

“It’s a definite buzz. Since the news [of the royal engagement] dropped, we’ve had a lot of women coming in and asking for either her cut or rich color,” says Osterman. One of the main components of the coif is half-coverage of the forehead, according to Osterman, who says the timing for the cut is perfect.

“In the last couple of years, it’s been all about the full bangs, and people are growing them out.”

Rob Shuter predicts Middleton’s hair will become the new “Rachel,” popularized by Jennifer Aniston’s character on “Friends.”

TWEEDY COATS

Kate’s tweedy English country style has sparked a topcoat craze in the States. Katherine Hooker, the custom coat designer behind at least three of Kate’s oft-worn jackets, has seen a tsunami of interest.

Hooker says the phone has been ringing off the hook at her London shop, and the number of hits on her Web site has shot up in the wake of the regal announcement.

“Normally we have one to five people register online per day, and since the engagement, we’ve had an average of 30 registrations a day,” says a spokesperson. In a serendipitous move, the designer — who hosted a monthlong trunk show throughout the States in October — has just appointed former Ralph Lauren designer Karimah Gottschalck to help expand the brand in this country. Starting in the new year, Hooker will add at least three months to her yearly schedule of US trunk shows. “In my eyes, [Kate] is a perfect customer,” Hooker tells The Post. “She’s beautiful, stylish, but still approachable, and that’s what we’re about.” Information on upcoming trunk shows can be found on her Web site, katherinehooker.com.